Detection of electrical defects on structures formed on a semiconductor wafer is indispensable for improving the productivity of devices. With the development trend in miniature devices, the ability to examine microscopic structures and to detect microscopic defects becomes crucial to the fabrication processes.
Various technologies and methods of defect inspection on structures formed on semiconductor wafers have been developed and employed with varying degrees of success. Optical inspection methods employ optical inspection tools, such as an optical microscope. One example of an optical inspection tool is KLA-Tencor's 29xx Series broadband plasma defect inspection system. This type of device usually involves collecting radiation emitted from a sample or scattered by a sample from an incident beam of radiation directed at the structure. The collected radiation is converted to signals that can be measured or used to form an image. Such measurements or images can be used to determine various characteristics, such as the profile of the structure. The optical inspection tool may also indirectly detect electrical defects caused by some physical or morphological detects either on the wafer surface or below the surface. While optical inspection can be performed fairly fast, it cannot detect subtle electrical defects caused by non-optically visual process issues such as grain boundaries. In addition, optical inspection suffers from a lack of spatial resolution, making it difficult to precisely identify defective features on the wafer.
Another method uses scanning electron microscopes (SEMs) for detection of electrical defects. Secondary charged particles are emitted from a surface of the specimen to be examined when electron or ion source impinges on the surface with sufficient energy. Since the energy and/or the energy distribution of such secondary charged particles offers information as to the topography of the specimen, SEMs are employed to detect the secondary charged particles and convert them to electrical signals used to generate images of the specimen for defect inspection. SEM inspection allows location of electrical defects with submicron position accuracy. KLA-Tencor eS8xx Series tools and Hermes Microvision eScanxxx Series tools are the examples of SEM tools that are widely used. However, SEM tools are insensitive to certain types of electrical defects.
Next, physical electrical probing of defects has been a standard part of quality control for integrated circuits. Existing “nanoprobing” tools allows spatial resolution of 5 nm. However, nano-scale physical probing is usually slow. Automation is difficult and the process can be highly labor-intensive. Thus, it is desirable to develop a fast inspection tool that is sensitive to transient and subtle voltage contrast defects.
It is within this context that aspects of the present disclosure arise.